The present invention relates to methods of thermoluminescence analysis and, more particularly, to versatile method and device for determining physico-chemical properties of materials using thermoluminescence analysis, which can be applied to geology, forensic science, industry and other branches of human activities. Examples of such applications may be: exploration of petroleum, natural gas, and minerals; forensic examination of objects; detection of environmental pollution; and production quality control.
The thermoluminescence phenomenon (TL) occurs widely in nature. TL occurs in a great number of various materials such as natural minerals, solid organic substances such as bones or fish's scale, and various rocks such as calcite, dolomite, feldspars, quartz and zircon. Physical processes underlying the thermoluminescence of solids are essentially well studied. The natural or artificial irradiation excites electrons from the valence band of a solid knocking them free from bonded (local) states. Most of the excited electrons return to the valence band after a short time producing a light emission (luminescence). Some of them, however, are trapped in local trapping levels within the forbidden band. Each electron coming out of the valence band leaves a hole in this band which may be as well trapped in a corresponding level within the forbidden band. These trap levels are usually associated with lattice defects, such as vacancies or impurities. The electron trapping levels are called traps, and those of hole trapping are called recombination centers.
Trapped (localized) charge carriers, electrons and holes, can remain in this state for a very long time at room temperature or below. However, at elevated temperatures (when heated) these charge carriers, such as localized electrons are released from the trap and, freely moving in the crystal, may recombine with holes trapped in recombination centers. If those recombinations are accompanied by light emission, thermoluminescence is observed, the properties of which largely depend on previous radiation and the physical and chemical conditions in which the sample was before.
The use of thermoluminescence (TL) for analyzing physico-chemical properties of materials is well established, see for example, S. W. S. McKeever "Thermoluminescence of Solids", Cambridge, Solid State, Science Series, Cambridge University Press, 1985.
Various attempts have been made to develop accurate and efficient methods and devices for determining the physico-chemical properties of materials using thermoluminescence analysis.
Prior art includes various methods and devices applicable to geology and mineralogy, such as methods and devices for studying meteorites, stalactites and volcanic rocks, and for dating and identification of archeological materials. Examples are disclosed in: S. W. S. McKeever "Thermoluminescence of Solids", Cambridge, Solid State, Science Series, Cambridge University Press, 1985. However, the main drawback of such methods and devices is that they are designed for a specific purpose, such as dosimetry, and therefore their use is relatively limited and expensive.
Further prior art includes methods and devices applicable to environmental monitoring, such as methods and devices for detecting local radioactive contaminations. Examples are disclosed in: Wachsmann F. and Regulla D. F. Kemtechnik 7, 318, 1978; Wachsmann F. and Regulla D. F. Proc. Syrup on the Natural Radiation Environment, Houston, 23-28 Apr. 1978, eds. T. F. Gesell and W. M. Lowder, Vol. 2, p. 1022, (US DOI, Washington, 1980). Such method is based on the use of a set of detectors positioned at the site of supposed contamination. Deviations from the radioactivity background in the detector's readings indicate that there is radioactive contamination in the area. To carry out the method, thermoluminescent dosimeters are used which are specified for a certain set of detectors. However, such methods and devices are ineffective. Further, they are limited to a specific purpose and do not allow, for example, the detection of non-radioactive substances.
Additional prior art includes methods and devices applicable to exploration of petroleum and minerals. An example, which is the closest to the claimed invention, is disclosed in PCT Application No. WO 87/04528. Such method includes thermoluminescence analysis of a crystalline sample, such as feldspar or quartz, for determining the proximity of the sample to a mineral or petroleum deposit. The method comprises first irradiating the sample with gamma-radiation. Thereafter irradiated samples are heated from an ambient temperature to an elevated temperature. Then their thermoluminescence intensities are measured at different temperatures and the thermoluminescence curves are related to thermoluminescence intensities of reference samples having known characteristics indicative of the proximity of explored minerals or petroleum. The samples may be previously treated to remove unwanted accessory minerals, such as feldspar and zircon, and shielded from direct light to allow phosphorescence and radioluminescence to decay.
However, the main drawback of this method is in the necessity to always irradiate samples before taking thermoluminescence curves which may hide the natural distribution of the radioactivity background in the analyzed area and, hence, necessary and significant information may be lost when making a conclusion on the presence or absence of explored petroleum or natural gas. Another drawback is the preparation of reference samples from materials subjected to different degrees of physical action (a certain dose of irradiation). Thus, in relating the analyzed samples to reference samples, only the intensity difference of certain TL peaks is studied. This excludes the evaluation of petroleum and natural gas influence on other physico-chemical properties of the samples causing the appearence of additional TL peaks or disappearence of old peaks thereof.
Finally, most of the disclosed methods and devices of TL-analysis are ineffective, inefficient, limited to a specific purpose, expensive, and require some special training of the investigator.
There is thus a widely recognized need for, and it would be highly advantageous to have, methods and devices of thermoluminescence analysis which are applicable to various purposes such as: exploration of petroleum, natural gas and minerals; forensic examination of materials, substances and articles; environmental monitoring; and production quality control.
It would be further advantageous to have such methods and devices which allow a simultaneous detection of different factors, such as changes in the chemistry and radioactivity of a soil sample, for predicting an earthquake.
Also it would be advantageous to have such methods and devices which are: effective and accurate; capable of detecting, retaining and processing relevant information; convenient for ready use by untrained personnel; efficient and inexpensive.